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Postharvest fungal decay in onion (Allium cepa L.) storage and the associated risks of Listeria monocytogenes

Kellee Britt: University of Florida

<div>In response to an extensive 2012 commercial recall of fresh-cut onions due to <em>Listeria monocytogenes</em>, the presence of field and short-term storage conditions conducive to postharvest decay was investigated as a possible contributing factor to pervasive contamination by <em>L. monocytogenes</em>. Following co-inoculation with recognized onion fungal pathogens, <em>Botrytis allii</em> and <em>Aspergillus niger</em>, the survival and growth potential of <em>L. monocytogenes</em> in the postharvest storage environment of whole onions, pre-processing was evaluated. <em>L. monocytogenes</em> was shown to survive, but not grow, on the surface of whole onions in optimal storage conditions of 2.5°C and 70% RH, and ambient air atmosphere, as well as in conducive-to-decay storage conditions of 25°C and 95% RH. Net growth was not observed, irrespective of incipient or advanced decay by <em>B. allii</em> or <em>A. niger</em> in those respective environments. <em>L. monocytogenes</em> populations inoculated into onion neck-tissue wounds could grow slightly, but not significantly at optimal storage conditions and sub-optimal storage conditions of 15°C and 75% RH, regardless of <em>B. allii</em> infections<em>. L. monocytogenes</em> could not grow at conducive-to-disease storage conditions, regardless of onion infection with <em>A. niger. B. allii</em> and <em>A. niger</em> did not exhibit a positive metabiotic interaction on <em>L. monocytogenes</em> survival or net growth when co-inoculated into wounds of whole onions. These results suggest that although raw onions may harbor <em>L. monocytogenes</em>, the pervasive environmental detection of the human pathogen and on product was likely the result of other factors associated with the receiving facility.</div>